Rensch's education and scientific work was interrupted by service in the German military during both World War I and World War II. His autobiography, published later in life, translates as 'Life of a biologist in a turbulent century'.[1]

In 1929 he published a classic book on species and speciation in which he proposed ideas which later were to become basic principles of taxonomy.[2] He recognised the significance of Moritz Wagner's idea of geographical species-splitting.

In 1947, Rensch published Neuere Probleme der Abstammungslehre: die Transspezifische Evolution.[3] This looked at how the evolutionary mechanisms involved in speciation might be extended to explain the origins of the differences between the higher level taxa. His work contributed to the rapid acceptance of the synthesis in Germany.[4]